13.1: Energy and ATP - Biology

Learning Objectives

  • Describe the different types of energy.
  • Describe the structure and function of ATP.

Understanding photosynthesis and aerobic cellular respiration relies on the fundamentals of energy. Energy is defined as the ability to do work, and there are several types of energy (Figure (PageIndex{1})). Kinetic energy is the energy of motion. Examples include a ball rolling down a hill, heat energy, and light energy. Heat energy is technically energy that is transferred between systems without doing work. The higher the temperature, the faster molecules in matter move. Potential energy is the energy that matter possesses but is not currently being used. For example, a ball sitting a the top of the hill that has not yet rolled down the hill possesses potential energy. Chemical energy is an example of potential energy that is stored in molecules. When molecules that are higher energy and less stable react to form products that are lower energy and more stable, this stored energy is released.

Figure (PageIndex{1}): Water stored at the top of a dam has potential energy (left). Water rushing down an incline has kinetic energy (right). Image by OpenStax (CC-BY). Access for free at

Adenosine triphosphate (ATP) is considered the energy currency of the cell because it provides usable energy. Structurally, ATP resembles a modified nucleotide (the building blocks of DNA and RNA). Specifically, it consists of adenine, ribose, and three phosphate groups (Figure (PageIndex{2})). The bonds between the phosphate groups are unstable. When these bonds are broken, more stable bonds are formed in their place, releasing energy. Phosphorylation refers to adding a phosphate group (PO43-) to a molecule. However, it often refers specifically to synthesizing ATP by adding a phosphate group to adenosine diphosphate (ADP).

Figure (PageIndex{2}): With energy input, adenosine triphoshate (ATP) can be synthesized from adenosine diphosphate (ADP) and a phosphate group. ATP can then be broken down to provide usable energy to the cell. Image by OpenStax (CC-BY). Access for free at

13.1: Energy and ATP - Biology

Glycolysis is the process whereby glucose is converted to pyruvate in ten enzymatic steps. This process is catabolic i.e., it involves breakdown of a molecule into smaller pieces, and as is typical of catabolic processes, it results in the net production of ATP. There isn't a lot of ATP produced in glycolysis: just two molecules of ATP are produced per molecule of glucose input. Much more ATP is produced in the Krebs cycle steps that we will study in a couple of days. But since pyruvate is an essential starting point in that cycle, the process we're describing here leads the way to that energy-rich process.

Pyruvate is also a precursor to fatty acids and other metabolites, so the conversion of glucose to pyruvate has significance in that regard as well as its role in energy generation. Furthermore, the process produces two molecules of reduced NAD per input glucose molecule, so there is reducing power as well as energy generated in these steps.

Glycolysis includes some phosphorylation steps, which require energy. Thus the path from glucose to pyruvate is not all downhill some steps require ATP, whereas others liberate ATP. The net result, though, is release of two molecules of ATP per glucose:
Glucose + 2 ADP + 2 NAD + + 2P i -> 2 Pyruvate + 2 ATP + 2 NADH + 2H + + 2H 2 O
The table below is a summary of the reactions involved. Note that a central step in the process, the one catalyzed by aldolase, involves converting a 6-carbon bisphosphorylated sugar into two 3-carbon phosphorylated sugars. This is a typical catabolic reaction for saccharides. In the table, "E.C. number" refers to the enzyme commission code for the enzyme "Resolution" refers to the highest (or nearly the highest) resolution structure available for the protein in question "PDB code, yr" refers to the Protein Data Bank accession code for that highest-resolution structure, and the year in which that structure was submitted.

Enzymes in the Glycolytic Pathway

Products E.C.
PDB code,
# su
1CZA 1999
ATP, Mg 2+
1K2Y 2001
Zn 2+
1IAT 2001

557 2
fruc 1,6-bisP
1PFK 1988
ATP, Mg 2+
glyc3-P, DHA-P
1ADO 1996

1YPI 1991

Glyceraldehyde-3-P dehydrogenase
1,3-bisP glya
1GD1 1987
1,3-bisP glya
16PK 1998
ATP, Mg 2+
Phosphoglycerate mutase
1E58 2000
249 1-4
1ONE 1995
Mg 2+
Pyruvate kinase
1E0T 2000
ATP, Mg 2+

subunit (monomer)
phosphate, phospho-
adenosine triphosphate
nicotinamide adenine dinucleotide

Some of the enzymes have names that are emblematic of the reverse reactions, not the reactions as written here, namely, phosphoglycerate kinase and pyruvate kinase.

To really get a sense of what is happening in these reactions, you should look at the structures of the small molecules involved in each of these steps. This graphic is taken from a website at the University of Texas:

Glycolysis is characteristic of catabolic pathways for sugars in that it breaks a 6- (or, in other instances, 5-) carbon sugar down into two approximately equal-sized parts. The actual carbon-carbon bond breakage occurs at the aldolase step the other steps involve phosphorylations, dephosphorylations, and redox reactions. The enzyme ribulose bisphosphate carboxylase / oxygenase (RuBisCO) is part of an analogous pathway. It disrupts a carbon-carbon bond in a doubly phosphorylated sugar (similar to fructose 1,6-bisphosphate in glycolysis) to produce either a three-carbon sugar and a two-carbon compound or two three-carbon sugars:
ribulose 1,5-bisphosphate + O2 -> 2-phosphoglycolate + 3-phosphoglycerate + 2 H +
ribulose 1,5-bisphosphate + CO2 + H2 O -> 2 3-phosphoglycerate + 2H +
The first of these reactions is part of photorespiration, i.e. the consumption of oxygen in photosynthetic leaves. The second actually fixes--that is, pulls from the air or water--inorganic carbon in the form of carbon dioxide or bicarbonate. It is the principal source by which carbon is incorporated into molecular skeletons. We'll study these reactions in greater detail in chapter 15, but we note now the similarity in terms of the sugar bisphosphate's fate to that found in the aldolase reaction.

Why it's important

  1. Energy in the form of ATP this is used as fuel for many other reactions.
  2. Reducing power in the form of NADH this is required for oxidation-reduction reactions.
  3. Pyruvate, which is a significant starting point both for the Krebs cycle and for lipid biosynthesis.

The ten enzymatic steps

    Hexokinase transfers the γ-phosphoryl group of ATP to the oxygen atom at C-6 of glucose, producing glucose 6-phosphate and ADP. This is an instance where the coupling between ATP hydrolysis and an energy-requiring reaction is very close, because the phosphate is transferred directly from ATP to the recipient molecule, in this case glucose. Most enzymes that carry out a reaction of this kind have the word "kinase" at the end of their name.
    The reaction catalyzed by hexokinase is energetically favored:
    ΔG 0

-5.33 kcal/mol, so at 310K (human body temperature)
Keq = exp(-ΔG 0 /RT)
= exp(5.33 kcal/mol / [(1.987 * 10-3 kcal./mol-deg) * 310deg)]
= exp(5.33/(1.987*0.31)) = 5700, so under conditions of adequate ATP, the equilibrium will definitely favor the product (glucose 6-phosphate) over glucose.

The mechanism of the reaction catalyzed by phosphoglycerate mutase involves formation of 2,3-bisphosphoglycerate via transient phosphorylation of a histidine residue of the enzyme. 2,3BPG can diffuse from phosphoglycerate mutase, however, leaving the enzyme trapped in an unusable state. Cells make excess 2,3BPG (using the enzyme bisphosphoglycerate mutase) in order to drive 2,3BPG back to phosphoglycerate mutase, so the reaction can go to completion.

The fate of pyruvate

If oxygen is abundant, pyruvate is ordinarily converted to acetyl coenzyme A, and that serves as an entry point into the tricarboxylic acid (citric acid, or Krebs) cycle. With oxygen available, the NADH that has been produced in the glyceraldehyde 3-phosphate dehydrogenase step becomes reoxidized to NAD with concomitant release of energy. We'll discuss that in detail next week. But if oxygen is scarce, a different pathway known as fermentation, in which pyruvate is converted to lactate, predominates.
The enzyme that catalyzes this conversion, lactate dehydrogenase, is a tetrameric, NAD-dependent enzyme with a molecular mass around 35kDA per subunit--that is, it is distinctly similar to glyceraldehyde 3-phosphate dehydrogenase. It catalyzes the reaction
pyruvate + NADH + H + <-> lactate + NAD
so the name is derived from the reverse reaction. An alternative name for this enzyme would be "pyruvate reductase". This is a zinc-dependent enzyme , and several structures have been determined for it.
In the absence of oxygen in yeast, a different pathway is followed.

Free energy in glycolysis

Examine carefully fig. 11.12 in Horton. The point it makes is that, although the standard free energies associated with the various reactions in glycolysis vary widely, the true free energy changes are monotonically negative and rather small as we go from glucose to pyruvate.In particular, there are really only three steps in the process that are effectively irreversible: the first, third, and last steps, i.e. the hexokinase, phosphofructokinase, and pyruvate kinase steps. All the others have &DeltaG values close to zero. So the only steps that are irreversible are the ones that involve formation or breakage of high-energy phosphate bonds. The difference between free energy and standard free energy is one we emphasized in the previous chapter. In this instance, the relative abundances of the various metabolites involved in glycolysis drives the reactions whose &DeltaG o ' values are positive toward the right.

Regulation of glycolysis

This brings up a related point: irreversible reactions tend to be the reactions for which control mechanisms come into play. Horton offers a description of hexose transporters, which are proteins involved in moving hexoses around from one cell to another. There are also control mechanisms that operate by inhibition of specific enzymes in the pathway. In glycolysis, the enzymes on which inhibitory controls are exerted are the three kinase steps discussed above.

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13.1: Energy and ATP - Biology

Photosynthesis takes place inside chloroplasts .

These are organelles surrounded by 2 membranes, called an envelope .

Chloroplasts are found in mesophyll cells in leaves:

- Palisade mesophyll cells contain most chloroplasts.
- Spongy mesophyll cells and Guard cells also contain chloroplasts.

The membranes inside a chloroplast are called lamellae , and it is here that the light-dependent
reactions take place. The membranes contain chlorophyl molecules, arranged in groups called photosystems . There are two kinds of photosysterns, PSI and PSII, each of which contains slightly different kinds of chlorophyll.

There are enclosed spaces between pairs of membranes, forming fluid-filled sacs called thylakoids. These are involved in photophosphorylation - the formation of ATP using energy from light. Thylakoids are often arranged in stacks called grana (singular: granum),

Stroma and light-independent reactions

The 'background material' of the chloroplast is called the stroma, and this is where the light-independent reactions take place.

Chloroplasts often contain starch grains and lipid droplets. These are stores of energy-containing substances that have been made in the chloroplast but are not immediately needed by the cell or by other parts of the plant.


As the connections among different cell compartments and the ways of corporation between different metabolic pathways in eukaryotic organisms could be many and varied, their carbon and energy metabolisms are complex and fascinating [29, 34, 35, 37,38,39]. Recent results showed that diatoms optimize their photosynthetic efficiency via elaborate interactions between plastids and mitochondria [41]. Besides, several studies have also noticed the synergistic photosynthesis and glucose metabolism in certain microalgae species under mixotrophic cultivation, but its carbon and energy metabolic mechanism is currently poorly understood [7, 9, 11]. The present study provided an interaction scenario between photosynthesis and glucose metabolism in C. zofingiensis under mixotrophic cultivation, in which the additional organic carbon source can replace the RuBisCO-fixed CO2 for the organic carbon metabolism in the chloroplast, and provides sufficient precursors for the utilization of light energy. Thus, rather than CO2 fixation, photosynthesis became mainly employed for light energy fixation in mixotrophy. This was similar with previous results in cyanobacteria, where the authors indicated that the photosystems are mainly employed for reducing equivalents and energy supplies with limited CO2 fixation during mixotrophic growth [42]. As a result, the photosynthesis rate-limiting enzyme, RuBisCO, was skipped in mixotrophy, which could reduce energy waste of photosynthesis while promote light energy utilization efficiency and cell growth. And similar results that mixotrophy can confer a higher growth rate than the sum of photoautotrophy and heterotrophy have also been reported in other microalgal species [7, 43].

Glucose and its metabolic intermediates and ATP were presented both in photosynthesis and glucose metabolism, which could also function as regulators in many biological processes, and might coordinate photosynthesis and glucose metabolism in mixotrophy. It was reported that chloroplast-derived carbohydrates could regulate cellular metabolisms [34]. For instance, triose phosphates can trigger the expression changes of cytosolic transcription factors, and organic carbons were reported to feedback regulate photosynthesis [44]. Besides, evidence have showed that post-translational regulation may affect the Calvin cycle enzymes in microalgal species [45]. Apart from the regulations of organic carbons on photosynthesis, the regulation of photosynthesis on glucose uptake has also been reported in the present study. Besides, citrate synthase, a key enzyme of TCA cycle, was reported to be regulated by the ratio of ATP/ADP [46]. As photoreaction could provide ATP for cell metabolism, it was not hard to understand the downregulation of TCA cycle in mixotrophy. In general, the mutual regulation of photosynthesis and glucose metabolism is a complex process. The mixotrophic metabolic mechanism proposed in this study is the result of their collaboration.

Previous research showed that, adg1-1/tpt-2, an Arabidopsis thaliana double mutant impaired in acclimation to high light with an 80–90% inhibition of ETR, could be rescued by exogenously supplied sugars (i.e., glucose and sucrose) [47, 48]. A scenario was proposed that the fed sugars would be transported into chloroplast and used for anabolism. However, a recent review pointed out that this scenario would entail the assumption that CO2 fixation by Calvin–Benson cycle would be minimized or even blocked through sugar feeding, which awaits to be tested experimentally [34]. The present study showed that the intermediates derived from exogenous glucose would directly enter the chloroplast and replace RuBisCO-fixed CO2 to provide carbon sources for chloroplast organic carbon metabolism in mixotrophy. Therefore, CO2 fixation was skipped, as reflected by a significant down-regulation of gene expression. And these results experimentally verified the above assumption is valid in C. zofingiensis and provide a reference for research in plants [47]. Many works have been done on directly engineering of RuBisCO to accelerate CO2 fixation rate [49, 50]. It was previously reported that under current atmospheric conditions, nearly 30% of the carbohydrates formed by C3 photosynthesis are lost via photorespiration [33, 51]. However, photorespiration is indispensable for photosynthetic organisms, since this pathway participates in photoprotection [32], amino acid biosynthesis [52] and removal of toxic intermediate metabolites [53]. Hence, reducing rather than eliminating photorespiration has become an attractive avenue for improving photosynthetic efficiency [26, 33, 51, 54]. Recent work has shown that re-engineering photorespiratory pathways can significantly increase biomass production in higher plants [55]. The present study has been the first to show that skipping RubisCO could significantly reduce NPQ and photorespiration, and provided a strong evidence that increase of light energy fixation can be achieved not only by directly increasing CO2 fixation or by modifying photorespiration [55], but also by skipping the photosynthesis rate-limiting steps. Collectively, this study not only elaborated the mixotrophic metabolic mechanisms of C. zofingiensis, but also provides a theoretical basis and new ideas for future research on photosynthesis and glucose metabolism, and provides a foundation for future industrial applications of mixotrophy.

Synergy between (13)C-metabolic flux analysis and flux balance analysis for understanding metabolic adaptation to anaerobiosis in E. coli

Genome-based Flux Balance Analysis (FBA) and steady-state isotopic-labeling-based Metabolic Flux Analysis (MFA) are complimentary approaches to predicting and measuring the operation and regulation of metabolic networks. Here, genome-derived models of Escherichia coli (E. coli) metabolism were used for FBA and ¹³C-MFA analyses of aerobic and anaerobic growths of wild-type E. coli (K-12 MG1655) cells. Validated MFA flux maps reveal that the fraction of maintenance ATP consumption in total ATP production is about 14% higher under anaerobic (51.1%) than aerobic conditions (37.2%). FBA revealed that an increased ATP utilization is consumed by ATP synthase to secrete protons from fermentation. The TCA cycle is shown to be incomplete in aerobically growing cells and submaximal growth is due to limited oxidative phosphorylation. An FBA was successful in predicting product secretion rates in aerobic culture if both glucose and oxygen uptake measurement were constrained, but the most-frequently predicted values of internal fluxes yielded from sampling the feasible space differ substantially from MFA-derived fluxes.

Biochemistry. 5th edition.


The flow of ions through a single membrane channel (channels are shown in red in the illustration at the left) can be detected by the patch clamp technique, which records current changes as the channel transits between the open and closed states. [(Left) (more. )

The lipid bilayer of biological membranes, as discussed in Chapter 12, is intrinsically impermeable to ions and polar molecules. Permeability is conferred by two classes of membrane proteins, pumps and channels. Pumps use a source of free energy such as ATP or light to drive the thermodynamically uphill transport of ions or molecules. Pump action is an example of active transport. Channels, in contrast, enable ions to flow rapidly through membranes in a downhill direction. Channel action illustrates passive transport, or facilitated diffusion.

Pumps are energy transducers in that they convert one form of free energy into another. Two types of ATP-driven pumps, P-type ATPases and the ATP-binding cassette pumps, undergo conformational changes on ATP binding and hydrolysis that cause a bound ion to be transported across the membrane. Phosphorylation and dephosphorylation of both the Ca 2+ -ATPase and the Na + -K + -ATPase pumps, which are representative of P-type ATPase, are coupled to changes in orientation and affinity of their ion-binding sites.

A different mechanism of active transport, one that utilizes the gradient of one ion to drive the active transport of another, will be illustrated by the sodium�lcium exchanger. This pump plays an important role in extruding Ca 2+ from cells.

We begin our examination of channels with the acetylcholine receptor, a channel that mediates the transmission of nerve signals across synapses, the functional junctions between neurons. The acetylcholine receptor is a ligand-gated channel in that the channel opens in response to the binding of acetylcholine (Figure 13.1). In contrast, the sodium and potassium channels, which mediate action potentials in neuron axon membranes, are opened by membrane depolarization rather than by the binding of an allosteric effector. These channels are voltage-gated. These channels are also of interest because they swiftly and deftly distinguish between quite similar ions (e.g., Na + and K + ). The flow of ions through a single channel in a membrane can readily be detected by using the patch-clamp technique.

Figure 13.1

Acetylcholine Receptors. An electron micrograph shows the densely packed acetylcholine receptors embedded in a postsynaptic membrane. [Courtesy of Dr. John Heuser and Dr. Shelly Salpeter.]

The chapter concludes with a view of a different kind of channel—the cell-to-cell channel, or gap junction. These channels allow the transport of ions and metabolites between cells.

  • 13.1. The Transport of Molecules Across a Membrane May Be Active or Passive
  • 13.2. A Family of Membrane Proteins Uses ATP Hydrolysis to Pump Ions Across Membranes
  • 13.3. Multidrug Resistance and Cystic Fibrosis Highlight a Family of Membrane Proteins with ATP-Binding Cassette Domains
  • 13.4. Secondary Transporters Use One Concentration Gradient to Power the Formation of Another
  • 13.5. Specific Channels Can Rapidly Transport Ions Across Membranes
  • 13.6. Gap Junctions Allow Ions and Small Molecules to Flow between Communicating Cells
  • Summary
  • Problems
  • Selected Readings

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ATP Structure

As indicated by the molecular name, adenosine triphosphate consists of three phosphate groups (tri- prefix before phosphate) connected to adenosine. Adenosine is made by attaching the 9' nitrogen atom of the purine base adenine to the 1' carbon of the pentose sugar ribose. The phosphate groups are attached connecting and oxygen from a phosphate to the 5' carbon of the ribose. Starting with the group closest to the ribose sugar, the phosphate groups are named alpha (α), beta (β), and gamma (γ). Removing a phosphate group results in adenosine diphosphate (ADP) and removing two groups produces adenosine monophosphate (AMP).

13.1: Energy and ATP - Biology

Neural control initiates the formation of actin – myosin cross-bridges, leading to the sarcomere shortening involved in muscle contraction. These contractions extend from the muscle fiber through connective tissue to pull on bones, causing skeletal movement. The pull exerted by a muscle is called tension. The amount of force created by this tension can vary, which enables the same muscles to move very light objects and very heavy objects. In individual muscle fibers, the amount of tension produced depends primarily on the amount of cross-bridges formed, which is influenced by the cross-sectional area of the muscle fiber and the frequency of neural stimulation.

Muscle tension: Muscle tension is produced when the maximum amount of cross-bridges are formed, either within a muscle with a large diameter or when the maximum number of muscle fibers are stimulated. Muscle tone is residual muscle tension that resists passive stretching during the resting phase.

Cross-bridges and Tension

The number of cross-bridges formed between actin and myosin determine the amount of tension that a muscle fiber can produce. Cross-bridges can only form where thick and thin filaments overlap, allowing myosin to bind to actin. If more cross-bridges are formed, more myosin will pull on actin and more tension will be produced.

Maximal tension occurs when thick and thin filaments overlap to the greatest degree within a sarcomere. If a sarcomere at rest is stretched past an ideal resting length, thick and thin filaments do not overlap to the greatest degree so fewer cross-bridges can form. This results in fewer myosin heads pulling on actin and less muscle tension. As a sarcomere shortens, the zone of overlap reduces as the thin filaments reach the H zone, which is composed of myosin tails. Because myosin heads form cross-bridges, actin will not bind to myosin in this zone, reducing the tension produced by the myofiber. If the sarcomere is shortened even more, thin filaments begin to overlap with each other, reducing cross-bridge formation even further, and producing even less tension. Conversely, if the sarcomere is stretched to the point at which thick and thin filaments do not overlap at all, no cross-bridges are formed and no tension is produced. This amount of stretching does not usually occur because accessory proteins, internal sensory nerves, and connective tissue oppose extreme stretching.

The primary variable determining force production is the number of myofibers (long muscle cells) within the muscle that receive an action potential from the neuron that controls that fiber. When using the biceps to pick up a pencil, for example, the motor cortex of the brain only signals a few neurons of the biceps so only a few myofibers respond. In vertebrates, each myofiber responds fully if stimulated. On the other hand, when picking up a piano, the motor cortex signals all of the neurons in the biceps so that every myofiber participates. This is close to the maximum force the muscle can produce. As mentioned above, increasing the frequency of action potentials (the number of signals per second) can increase the force a bit more because the tropomyosin is flooded with calcium.

13.1: Energy and ATP - Biology

Why the body needs food

Your metabolism is the collection of chemical reactions that occur in your cells to sustain life. Some of these reactions use stored energy to build things up, which we call anabolism, while other reactions break things down, releasing energy that can be stored for future use, and this is called catabolism. Imagine that the hamburger you’re having for dinner, made of proteins, fats, and carbohydrates, is a collection of lego blocks of various colors and shapes. It took a lot of energy to organize those blocks into that complex structure, and breaking the blocks apart releases that energy and frees the blocks so that they can be built back up into new things. Your body does exactly that when you eat your food. Here's a brief video lecture that summarizes this concept.

Living things break down the three major categories of foods (proteins, fats, and carbohydrates) into their constituent parts, the individual lego blocks, for two reasons. 1) Once the food atoms and groups of atoms (molecules) are broken down, they can be built back up into the specific kinds of things the organism needs, like bone, muscle, skin, hair, feathers, fur, bark, leaves, etc. 2) Breaking down the food molecules releases the energy that was holding them together, and that released energy is temporarily stored by the cell for the re-building process. Each of these food types requires a different breakdown process, and we’ll look at those later, but the goal is the same–take the energy that held those food molecules together and release it so that it can be stored in a form that the cell can use later to build what it needs. The cell has a special kind of molecule for storing that energy, and it’s called ATP.

ATP (Adenosine tri-phosphate) is an important molecule found in all living things. Think of it as the “energy currency” of the cell. If a cell needs to spend energy to accomplish a task, the ATP molecule splits off one of its three phosphates, becoming ADP (Adenosine di-phosphate) + phosphate. The energy holding that phosphate molecule is now released and available to do work for the cell. When the cell has extra energy (gained from breaking down food that has been consumed or, in the case of plants, made via photosynthesis), it stores that energy by reattaching a free phosphate molecule to ADP, turning it back into ATP. The ATP molecule is just like a rechargeable battery. When it’s fully charged, it’s ATP. When it’s run down, it’s ADP. However, the battery doesn’t get thrown away when it’s run down–it just gets charged up again.

Here’s what it looks like chemically. Each phosphate is a PO4 (oxygen has a charge of -2 and there are 4 of them, for a total of -8, and P has a charge of +5, so the net charge on the phosphate group is -3. If free H atoms, which are +1, get added to the O atoms that aren’t bonded to two things, then the net charge is zero.)

There are times when the cell needs even more energy, and it splits off another phosphate, so it goes from ADP, adenoside di-phosphate, to AMP, adenosine mono-phosphate.

ATP ß à ADP + P + energy ß à AMP + P + energy

There are other energy storage molecules in the cell, like NAD and FAD, but the ATP system is the most common, and the most important. Think of the others as different brands of rechargable batteries that do the same job. Next, we’ll explore some of the pathways that the body uses to break down foods of different types.

What about oxygen? Why do we need that? What happens if you put a glass over a candle? You starve the fire of oxygen, and the flame flickers out. If a metabolic reaction is aerobic, it requires oxygen. Buy why? Here's an analogy. Think about lighting a campfire. What do you need? You need fuel (the wood), you need heat (it's harder to light a fire when it's cold), and you need oxygen (because another word for burning is "oxidizing" and, as you might guess, it can only occur in the presence of oxygen). Oxidizing something causes it to lose electrons, which means that energy (the electrons) is released when you oxidize, or burn, a fuel. Your food is your fuel. You burn the fuel for energy. You need the oxygen to burn the fuel. This happens in the mitochondria.